[mixpanel] Expose additional initialization options

There are certain use cases where the default initialization options for the mixpanel client won't be sufficient: for example, [implementing mixpanel in the EU](https://developer.mixpanel.com/docs/implement-mixpanel#implementing-mixpanel-in-the-european-union-eu), which requires a different `api_host`.

It would be great to allow users to specify these in the environment config and pass them through to [mixpanel.init](https://github.com/adopted-ember-addons/ember-metrics/blob/master/addon/metrics-adapters/mixpanel.js#L21)

Looks like #229 and/or #244 might address this issue.